1. Field of the Invention
The present invention relates to a mixing apparatus and, more particularly, to a mixing apparatus for efficiently performing such mixing operations as mixing, dissolution, crystallization, reaction, and slurry suspension.
2. Description of the Prior Art
Various types of impellers have been used in mixing tanks for various applications. When it is required to handle liquid over such a wide viscosity range that a flow condition in a tank can change from turbulent to laminar, there have often been used multiple stage impellers of a two-bladed flat paddle type. In most multiple impellers of this type, two impellers, adjacent each other on a rotating shaft are set crosswise, at right angles to each other, because this arrangement is the most stable from the viewpoints of rotational balance and shaft-bending moment caused from fluctuating hydrodynamic forces acting on impeller blades. Multiple impellers of this type usually have a comparatively wide axial clearance between two impellers adjacent to each other in order to decrease the number of impellers required and total power consumed by all the impellers in a tank.
In the meantime mixing apparatuses have been recently demanded to have more efficient and diversified performances so as to be applied to a wide range of mixing operations.
Particularly in batch processes, mixing apparatuses are required to meet the following diversified purposes and high performances.
(1) Uniform mixing; Complete mixing of liquids can be attained over a wide range of viscosity. And complete and uniform mixing can be attained under the conditions of low to high liquid level at various fluid viscosities.
(2) Heat-transfer performance; High heat-transfer coefficients with low power consumption is obtainable.
(3) Solid-liquid mixing; Various types of solid-liquid mixing such as dispersion of higher density particles, uniform mixing of high-concentration slurry, and uniform dispersion by low-shear agitation can be attained.
(4) Liquid-liquid dispersion; Sharp droplet size distribution is obtainable. In addition it is possible to attain uniform liquid-liquid dispersion by low-shear agitation and to attain homogeneous light-liquid dispersion whose viscosity is largely increased through reactions.
The present inventors investigated using conventional mixing apparatuses equipped with multistage impeller systems whether they could satisfy the aforementioned demands. The results of the investigations showed that it is not so difficult to choose a mixing apparatus capable of satisfying any one of the above-described demands. However, it is very difficult to select an apparatus specification capable of simultaneously satisfying many items of the aforementioned demands, since the prior-art mixing apparatuses have the following problems. Accordingly, the use of a mixing apparatus newly improved or developed will become necessary in order to meet the aforementioned demands.
In a mixing apparatus equipped with two-bladed paddle impellers multiply set on a shaft at the crossing angle of 90 degrees, which is described above as a prior-art or conventional type, streamlines are formed around individual impellers by their pumping action but do not sufficiently link each other between different stages of impellers. This can cause separation of flow regions of upper and lower stages, and stagnant points (dead spaces) between them near the tank wall, and these phenomena reduce the efficiencies of mixing performances. The formation of the dead spaces not only reduces heat-transfer performance but sticking and contamination are liable to be caused on the inner wall surface of the tank, and therefore it can adversely affect mixing results.
When flat-blade impellers are used in a multiple paddle impeller structure, a number of separate circulating flows are generated in a tank by the rotation of the impellers. This phenomenon becomes more typical when a clearance L between upper and lower impellers is kept as large as shown in FIG. 6(a). In such a case, since a boundary B is generated by a kind of weak interaction of two circulating flows, mixing between the upper and lower circulating flows is suppressed. On the contrary, the reduction of the clearance L between the impellers by moving them might be considered effective so as to avoid this weak interaction between circulating flows. In this case, however, it is necessary to increase the vertical dimension of either impeller by the same amount as the reduction (h', shown in the FIG. 6(b)) in the impeller clearance L, and this causes an incremental increase in power consumption. Also, in the case of increasing the impeller vertical dimension, the discharged flow is divided at the mid-height of the impeller, and two circulating flows and another boundary B' between them are generated as shown in FIG. 6(b), and thus the result is similar to that in the case of impellers attached with a wide clearance as described above. These phenomena disturb the linkage of circulating flows in the tank.
However, the optimum crossing angle and clearance between each impeller from the viewpoint of overall flow pattern in a tank have not been examined theoretically and quantitatively so that it is difficult to select the apparatus specification for satisfying the above-described demands.